1. Field of the Invention
The present invention generally relates to video cameras and, more particularly, is directed to a video camera in which an aperture of a high brightness portion can be emphasized.
2. Description of the Prior Art
In conventional video cameras, a white portion (high brightness portion) is compressed in order to extend a dynamic range of an imager signal.
FIG. 1 of the accompanying drawings shows a diagram of a conventional white compressing circuit.
Referring to FIG. 1, a video signal of a picked-up image is supplied to a video signal input terminal 1, and the video signal applied to the input terminal 1 is supplied through a resistor R1 to an output terminal 2. A junction 10 between the resistor R1 and the output terminal 2 is coupled to the anode of a diode 4, and an output of a reference voltage source 4 is supplied through a variable resistor R2 to the cathode of a diode 3.
Thus, when the video signal applied to the input terminal 1 becomes higher in level than the output level of the reference voltage source 4, the diode 3 is turned on and a gain G of the video signal developed at the output terminal 2 is expressed by the following equation (1): EQU G=R2/(R1+R2) (1)
If the gain G is lowered as shown by the above equation (1), then the high brightness portion is compressed. FIGS. 2A and 2B are respectively diagrams showing the state such that the white portion is compressed. When a video signal whose brightness is progressively increased as shown in FIG. 2A is supplied to the input terminal 1 of the white compression circuit shown in FIG. 1, if the level of the input video signal exceeds a constant level x determined by the output potential of the reference voltage source 4 as shown in FIG. 2B, then the gain is lowered and the high brightness portion is compressed. Incidentally, the point x in FIG. 2B is what might be called a "knee point".
In the ordinary video cameras, an aperture of the video signal of the picked-up image is frequently emphasized. In this case, the above compression of the high brightness portion is effected after the aperture is emphasized so that, when the high brightness portion is compressed as described above, an aperture correcting signal (aperture emphasizing signal) added to the video signal also is compressed. There is then the disadvantage such that the aperture of the high brightness portion becomes obscure.
More specifically, if an input video signal shown in FIG. 3A is obtained, then an aperture correction signal (see FIG. 3B) which rises at timing point in which the brightness level of this video signal is changed is generated, and this aperture correcting signal is mixed into the video signal shown in FIG. 3A, thereby obtaining a mixed signal whose aperture is emphasized as shown in FIG. 3C is obtained. If this mixed signal is supplied to the white compression circuit, then a compressed signal shown in FIG. 3D is obtained. While the aperture correcting signal is not changed in the low brightness portion where the compressed signal is not compressed, the length of the leading edge of the aperture correcting signal also is compressed in the compressed high brightness portion so that the aperture emphasizing effect can be made useless substantially. To solve this problem, such a processing is proposed that the aperture correcting signal is added to the high brightness portion of the video signal output from the white compressing circuit one more time.
FIG. 4 of the accompanying drawings shows an arrangement of a white compressing circuit and other relating elements which are used to add the aperture correcting signal to the high brightness portion one more time.
As shown in FIG. 4, a video signal (imager signal) into which the aperture correcting signal is already mixed is supplied to an input terminal 11. Then, the video signal applied to the input terminal 11 is supplied to a white compression circuit 12, in which the high brightness portion of the video signal supplied thereto is compressed. The video signal thus processed is supplied to a mixer 13. In this case, a reference voltage signal is supplied to the white compression circuit 12 from a reference voltage generator circuit 14 and thereby the high brightness portion higher than the level of this reference voltage is compressed. Further, the video signal applied to the input terminal 11 is supplied to a slicing circuit 15 and only the video signal of the brightness portion to be compressed is extracted by this slicing circuit 15. Also in this case, the reference voltage signal is supplied to the slicing circuit 15 from the reference voltage generator circuit 14, and thereby only the brightness portion higher than the level of the reference voltage is extracted. The video signal of the high brightness portion extracted by the slicing circuit 15 is supplied to an aperture correcting signal generator circuit 16 which derives an aperture correcting signal of high brightness portion. Then, the aperture correcting signal of the high brightness portion is supplied to the mixer 13, in which the aperture correcting signal of the high brightness portion is mixed with the video signal having the compressed high brightness portion supplied to the mixer 13 from the white compression circuit 12, and a mixed signal is supplied to an output terminal 17.
Thus, when a signal shown in FIG. 5A is supplied to the input terminal 11 as the video signal of the image, for example, the slicing circuit 15 extracts its high brightness portion shown in FIG. 5B and the aperture correcting signal generator circuit 16 generates an aperture correcting signal (see FIG. 5C) of only the brightness portion. Then, the aperture correcting signal of only the high brightness portion is mixed into a video signal (FIG. 5D) whose high brightness portion is compressed by the white compression circuit 12 in the mixer 13 to obtain a mixed signal shown in FIG. 5E, whereby an aperture information of the high brightness portion is restored.
Since the aperture information of the high brightness portion is restored as described above, at least the aperture of even the high brightness portion is added and hence the image information of the high brightness portion is maintained at minimum. However, the circuit configuration shown in FIG. 4 is complicated and hence the development of a circuit which can emphasize the aperture of the high brightness portion with ease has been requested so far.